Growing numbers of people are lured to cities by the promise of lucrative work and a higher standard of life. But urban dwellers also bear the brunt of high pollution levels and associated disease. Use the map and charts below to compare cities around the world.
Less than 1% of homes in Addis Ababa are connected to a mains water supply, while in Ho Chi Minh City more than 70% of people drive to work. Click on the map for more.
Much nitrogen dioxide comes from vehicle exhausts. It is linked to increased susceptibility to respiratory infections, especially among asthmatics. The gas also reacts to form ozone, which is harmful in the lower atmosphere - although the ozone layer protects us at higher levels.
As industrialisation moved around the globe, the world’s top five cities were all located in the north. But by 2015 they will be overtaken by modern megacities, mainly in the developing world.